Electrovalves are used to control the driving of hydraulic pistons, which electrovalves may be driven directly or piloted (servocontrolled), the former being used to control relatively small flows whereas the latter are used for sizeable flows.
Piloted valves are normally used to drive hydraulic brakes, clutches and brake-clutches in presses and similar machines due to the need to obtain high response speeds requiring a fast evacuation of the oil flow displaced by the drive pistons.
Furthermore, in presses and similar machines in which the physical well-being of the operator that supplies the machine is in danger, the brakes and brake-clutches used to drive machines of this kind are also provided with double electrovalves, mounted in such a way that it is necessary for both to be simultaneously driven in order for the machine to be started, and the operation of one such electrovalve must suffice to cause the machine to stop, thereby offering a double stopping safety.
For safety purposes, before starting the next operating procedure with the machine, it is necessary to verify whether both valves worked properly in stopping in order thereby to once again have the double drive safety, which verification is frequently made by inductive detectors that report on the return of the electrovalve rod to its initial position, the electric information supplied by said detectors being processed in order for starting system operation to be locked in the absence of a signal.
Hydraulic brakes, clutches and brake-clutches for driving presses and similar machines are moreover frequently provided with systems for cushioning engagement and/or braking in order to avoid sudden machine operating procedures which may result in the machines breaking down, to which end a system is used, inter alia, consisting of providing the clutch thrust piston with a disk pushed by a number of auxiliary springs that provide less force than that produced by the actual piston, and with limited travel. Similarly, the brake side is provided with a disk pushed by other auxiliary springs that provide less force than the normal braking springs, the foregoing in order for the auxiliary springs to push a disk located on either the clutch side or the brake side, which disk will in either case face a packet of sheets, in one case the clutch sheets and in the other the brake sheets, the disk being in either case pushed, in accordance with the operation of the main piston, against the respective packet of sheets, in order to cause a smooth engagement and/or braking.
There is another way of controlling a smooth cushioning or pushing in said engagement and/or braking operations, as described in this same applicant's Spanish patent of invention 9400215 in which the thrust piston is divided into two parts, one being deemed the main piston and the other one the auxiliary piston, the latter providing less surface to the pressure fluid than the main piston, and therefore at the time of engagement the auxiliary piston will make a first approach to the clutch disk without actually contacting it, followed, upon the brake being released, by a first engagement phase in which the auxiliary piston approaches the main piston and a second phase, after contact between both pistons, in which the assembly works conventionally as a single piston.
Now then, in addition to the foregoing, the smooth braking system needs to be overridden at a given point in time in order that the machine may be stopped in an emergency, such as when the person handling the machine is in a danger zone and when a fast machine reaction is required without regard for its wholeness.